1. Field of the Invention
This invention relates to novel N-(2,6-disubstituted 4-pyridyl)phenylureas and phenylthioureas, agricultural composition comprising such active ingredients and methods of treating plants therewith.
2. Brief Description of the Prior Art
In our co-pending application, Ser. No. 947,468 filed Oct. 2, 1978, now U.S. Pat. No. 4,193,788 it is disclosed that N-(2-chloro-4-pyridyl)-N'-phenyl ureas accelerate or retard plant growth when used in very small amounts in the agricultural and horticultural fields and are very useful as so-called plant growth regulators which control growth of plants. These urea compounds are specifically grouped into substances collectively referred to as cytokinin-like substances represented by .sup.6 N-benzyladenine, kinetin, N-(4-pyridyl)-N'-phenylurea, etc.
Plant growth regulators exhibiting cytokinin-like hormonal activity (hereafter referred to as cytokinin hormone activity) can accelerate plant growth when used in very small amounts. On the other hand, plant growth can sometimes be suppressed when such chemicals are employed in excess amounts, i.e., in amounts over that effective to accelerate plant growth (sometimes referred to as overdose use).
Accordingly, while the terms "plant growth regulation" and "plant growth regulator" used herein refer primarily to acceleration, they sometimes refer to suppression of plant growth (in overdose use) in a broad sense. Such seemingly contrary activities by the plant growth regulator are characteristic of cytokinin activity. In this regard, herbicides for which cytokinin activity has clearly been established are not known, though some herbicides have been shown to have auxin activity.
In addition to the above-described aromatic urea compounds, biological activities of N-(2-substituted-4-pyridyl)-N'-phenylurea compounds have been investigated by the present inventors (Ser. No. 62,850 filed Aug. 1, 1979 now U.S. Pat. No. 4,279,639). As a result, it has been found that requirements in chemical structure for exhibiting cytokinin activity reside in the N-(4-pyridyl) moiety, N'-phenyl moiety and the pyridine ring substituted with specific substituents such as a halogen atom, a methoxy group, an amino group, a cyano group, a trifluoromethyl group or the like at the 2-position thereof, in addition to the requirement that plant growth regulators be derived from urea or thiourea. Previous understanding was that 2,6-disubstituted compounds would cause a reduction in activity due to their steric hindrance since it has been heretofore established that compounds having the 2-alkyl-substituted pyridine moiety show activity only equivalent to compounds having no substituents thereon and further that 2,6-dialkyl-substituted compounds provide markedly reduced cytokinin activity.
On the other hand, the inventors have found, during the course of developing a more practical process for preparing N-(2-chloro-4-pyridyl)-N'-phenylurea, that N-(2,6-dichloro-4-pyridyl)-N'-phenylurea which was believed to be useful as an intermediate for the synthesis of a series of plant growth regulators discovered by the inventors shows unexpectedly potent cytokinin activity, notwithstanding the fact that the latter is disubstituted at the 2- and 6-positions thereof.